The ability to self-regulate behavior is one of the primary developmental tasks in early childhood, serving to modulate emotional arousal, support social relationships, and enable readiness for learning. The physiological underpinnings of behavioral regulation are established during the prenatal period, a time when the architecture and neural scaffolding of the human brain is erected in the brief 266 days of human gestation and structural development occurs in tandem with functional gains. The rapid rate of development during this period underscores the sensitivity of the fetus to organizing influences. As the maternal endocrine state is dramatically altered across gestation, there is great interest in the organizing role of maternal hormones on fetal neurodevelopment. To this end, non-physiological concentrations of maternal prenatal hormones have been termed endogenous functional teratogens. In particular, maternal prenatal sex steroids may act on the neural circuits that control behavior and exert sex-specific inhibitory effects on fetal endocrine activity. Parturition marks a time of hormonal transition. In the first months of life, there is a surge in infant sex steroid activity of unknown behavioral relevance; yet studies that examine concurrent measurement of sex steroids and behavior in infancy and early childhood on a single occasion show associations of elevated sex steroids with higher arousal, irritability, aggression, and impulsivity, all of which either support or are embedded in the capacity to effectively self-regulate behavior. Framed by developmental systems theory, two studies are proposed to examine: 1) the organizational effects of prenatal sex steroid exposure on neural circuits that support postnatal regulatory development; and 2) the dynamic interrelationship between organizational effects of prenatal sex steroid exposure and activational effects of the postnatal sex steroid surge on regulatory development. The first study leverages existing prospective longitudinal prenatal data to evaluate the prediction of self-regulation capacity at age 5. The second study examines the functional significance of the hormonal transition associated with birth on behavior, with a particular focus on the postnatal sex steroid surge. Dr. Voegtline is an ideal candidate to conduct the proposed research studies as she merges existing expertise in theoretical models and measurement of socio-emotional development in infancy and early childhood with recent training in neurobehavioral development of the fetus. During the K99 phase of the award, Dr. Voegtline will complete rigorous didactic and experiential training across four interrelated objectives: 1) build expertise in biomarkers of maternal-fetal-infant endocrinology; 2) become adept in newer technologies to measure fetal neurobehavior; 3) further advanced methodological training; and 4) gain additional laboratory leadership and management skills. These activities, under the guidance of sponsors Drs. Janet DiPietro and Doug Granger, will provide Dr. Voegtline with the final training needed to achieve her long-term career goal to secure a position as a faculty member in a developmental psychobiology department and to conduct a program of research on hormone-behavior associations spanning the prenatal to early childhood periods.